The invention described herein may be manufactured, used, and licensed by or for the Government of the United States without the payment to me of any royalties thereon.
The present invention relates generally to the measurement of mass loss rate of samples undergoing thermal decomposition caused by electrical heating, and more particularly to measurements where the forces exerted by the electrical leads on the sample are minimized through the use of frictionless contacts. Minimizing such external forces make the mass loss rate measurements more accurate. Such measurements are useful in determining flammability parameters of large size samples, typically 50 to 100 grams, of combustible materials.
Manufacturers of fire detectors for commercial aircraft are seeking to shorten detection times, improve reliability, and enhance specificity using multi-sensor arrays and advanced logic. To facilitate this effort in the area of aircraft cargo compartment fire detection, the Federal Aviation Administration (FAA) has developed and characterized a reproducible smoke generation source consisting of several pure polymers (plastics) found in articles of luggage. The typical test specimen, which is the subject of a separate patent application, is a 10xc3x9710xc3x971 cm plaque thermoformed from natural, unfilled plastic pellets by compression molding and containing an embedded Nichrome(trademark) heating wire. The level and duration of smoke production are determined by the electrical current to the heating wire in the specimen. Flaming combustion can be initiated at any time during the test using a spark igniter, with the rate of heat release controlled by varying the current to the heating wire. Using a fire calorimeter, a specimen can be tested to determine mass loss rate, heat release rate, smoke density and morphology, and the concentrations, over time, of various combustion gases in both flaming and non-flaming events.
Accurate mass loss measurements during the tests are not feasible, however, because the bending stiffness and thermal expansion of the electrical wires connecting the test specimen to the power source produce large spurious forces on the load transducer that continuously measures the mass of the sample. These forces are unrelated to the specimen mass loss history and have to be eliminated or minimized to accurately measure the mass loss rate.
Accordingly, it is an object of the present invention to provide a means and method for accurately measuring the mass loss rate of thermally decomposing materials where the decomposition event is caused by electrically heating the material.
It is a further object of the present invention to provide a means and method for accurately measuring the mass loss rate of thermally decomposing materials by supplying electrical power to the material without exerting outside forces on the material.
It is a still further object of the present invention to provide a means and method for measuring the mass loss rate of thermally decomposing materials by supplying electrical power to the material through frictionless electrical contacts.
Briefly, the present invention measures the mass loss rate of a thermally decomposing material that is heated by means of a heating element embedded in the material. External forces exerted on the sample by the electrical leads distort the mass measurement and produce inaccurate results. To minimize these external forces, the leads are connected to copper wires that extend downward into cups of liquid mercury contained in separate brass cups electrically connected to a power supply. The copper wires descending from the leads touch nothing but the mercury from which it receives the electrical current. The buoyant force of the mercury on the copper wire and the heat loss to the mercury and the copper leads are negligible.